Depending on the radiation being used, it may be necessary to maintain a particular distance of the transducer or transducers from the surface of the specimen. Furthermore, depending on the type of radiation being used, it may be necessary to maintain a particular angle with respect to the surface of the specimen. The required precision is commensurately higher when the surface curvature on the specimen is greater and the focusing of the transducers being used is stronger.
Arrangements consisting of a plurality of transducers are used for many testing tasks, for example searching for defects of different orientation, or to accelerate the test by testing a plurality of sites simultaneously. The transducers must in this case be precisely positioned and aligned with respect to one another, especially when the measurement positions need to be partially assigned to the transducers for precise defect localization, when there is a strong surface curvature and when using focused transducers, and/or when testing a plurality of sites simultaneously.
In practice, manufacturing tolerances and deviations occur when using the transducers, as below with reference to the example of ultrasound transducers:                The outgoing sound beam is not exactly concentric. Real sound transducers have offset and angle errors of the sound beam in relation to the nominal axis;        According to the manufacturing tolerances, the time of flight origin point of the sound varies in relation to the reference edge of the transducer, for example an end-stop on the front edge;        Owing to housing tolerances, variations of the transducer positioning may take place, for example lateral offset in the event of diameter variations. The transducers may be displaced in the longitudinal direction owing to imprecise fitting. The transducer positioning may change after replacement of a transducer.        
In the case of a plurality of sensors mounted on a common holder, an individually different error is encountered for each sensor. Even when a highly precise mechanism is employed for positioning and aligning the sensor holder, the accuracy with which the individual sensors are aligned is insufficient.
It is known to carry out the sensor positioning by mechanical contact with the surface, for example using rolls and slight pressure by means of a spring force. When using a plurality of sound transducers, the sensor frame may be configured in multiple parts so that each transducer is separately guided and positioned by the pressure on the surface. A disadvantage with this is that precise guiding by rolls is not possible for surfaces with a strong and varying curvature.
Furthermore, exact spatial assignment of the measurements is not possible when the transducers can move without measurement of the relative position, although this is scarcely still practicable even with two transducers. A disadvantage with guiding by rolls is furthermore the mechanical stress on the specimen surface, or the scanning speed limitation required in order to avoid damage or wear. Furthermore, guiding by rolls is scarcely practicable in the immersion technique.
The manufacturing tolerances can be reduced through selection of the transducers. Some of said error effects can thereby be reduced. A disadvantage with this is that only a limited improvement can be achieved. Another disadvantage is that a greater number of transducers are used according to the degree of selection, particularly when the transducers are selected in relation to a plurality of types of manufacturing tolerances, for example offset, tilt of the sound beam, diameter.
As an alternative to a rigid sensor holder in which all the sound transducers are fitted in a fixed fashion, a sensor holder may be designed with adjusters. A disadvantage, however, is that one adjuster for offset and another for the tilt must be provided for each sound transducer, so that the emitted sound beams of the transducers can be adjusted according to the desired setpoint alignment. The outlay and space requirement necessary for this are scarcely practicable. Under certain circumstances, in the case of a compact transducer arrangement, it is not even possible to produce all the adjustment devices. A plurality of parameters must furthermore be set, which leads to highly complex adjustment.
Another possibility is to use an array transducer instead of a plurality of individual transducers. With appropriate driving of the array transducer, it is possible to implement different sound incidence directions and different focusing. A disadvantage is the much greater outlay for production and driving, for example electronics and signal processing, of an array transducer in comparison with the individual transducer. 2-Dimensional setting of the sound incidence direction requires a 2D array transducer, which is usually impracticable owing to the high number of elements. For arrangements of different types of individual transducers—for example different frequency—another disadvantage is that they cannot be replaced by a common array transducer, i.e. this would entail a corresponding number of array transducers and corresponding outlay.